The invention relates generally to hygroscopic wicks and, more particularly, to fiber wicks configured for transporting analyte fluids in assay devices.
Various assay devices are known for use in the home, office, clinic, hospital or doctor's surgery for providing an analytical result which is rapid and which requires a minimal degree of skill and involvement by the user. Examples include test devices or assays for pregnancy and fertile period (ovulation) determination.
Typical assay devices comprise a housing, a reaction medium positioned in the housing, upon which the assay chemistry occurs, and a wick for collecting the liquid to be assayed and transferring it to the reaction medium. In general, the assay device should merely require that a collection portion of the device be contacted with a sample (e.g., a urine sample for pregnancy testing), and thereafter no further user actions are required. The sample is carried from the collection portion to the reaction medium by the wick. Observation of changes to the reaction medium or a substrate carrying the reaction medium provide an analytical result. Ideally, the analytical result should be observable within a matter of minutes following sampling.
The actual analytic techniques used to obtain the results typically determine the presence or absence of, and/or quantify the amount of various analytes in, tissues and fluids of organisms. Currently most diagnostic testing is done with blood, urine, fecal material, saliva, or tissue biopsy. Assay devices for collecting and reacting these materials can be used for, inter alia, pregnancy and ovulation tests, drug-of-abuse tests, and infectious disease tests.
One problem with standard assay devices, particularly those designed for self-testing in the home, is that the user may have difficulty determining whether enough of the analyte fluid or other material has been provided to the sample collector. This may be a particular problem with urine sampling devices because urine may be relatively colorless. One approach to solving this problem is to provide diagnostic assay devices with a color change indicator to signal that sufficient analyte-containing liquid (e.g., urine, saliva, plasma) has been collected by the sample collector. In wick-based devices, the color change generally indicates that the wicking component has been “wetted out” with the analyte fluid.
In such devices, however, the color change indication is based on the presence of a dye or colored substance that undergoes a change when wetted with water or other aqueous liquid. U.S. patent application Ser. No. 10/495,714 ('714 Application”), for example, describes sample collectors having wicking components (termed “bibulous members”) that contain a dye that changes color as the result of a pH change. In some disclosed examples, a dry wick contains the acid form of a pH-sensitive dye which, upon wetting by the analyte-containing solution, experiences a pH change to more basic conditions, resulting in a color change. The color change is said to be distinctly visible to the eye so as to indicate sufficient analyte solution is present to both conduct the diagnostic analysis and effect color change.
Other compounds can undergo a color change when wetted. For example, a commonly used indicator is cobalt chloride (CoCl2). Anhydrous cobalt chloride is blue. When it bonds with two water molecules, (CoCl2.2H2O), it turns purple. Further hydration results in the pink hexaaquacobalt(II) chloride complex [Co(H2O)6]Cl2.
These visual indications are the result of employing pH-sensitive dye molecules, such as phloxine B or bromophenol blue, along with pH altering reagents (acids or bases), or inorganic compounds. In either case, use of these chemical compounds may interfere with, or otherwise alter, the analyte itself or the diagnostic assay. This may reduce the sensitivity of the assay or lead to false negative or false positive results.
Another approach to sufficiency indication is to permanently color the wicking material or provide it with a permanently colored marker and cover the wick or marker with an additional material that is substantially opaque when dry and light transmittent when wet. The above-mentioned '714 Application discloses the use of such materials as sugar paste, nitrocellulose membranes, and nylon microporous membranes to cover an acetate strip affixed to the wicking material. This approach, however, may introduce additional uncertainty due to the separation of the cover material from the wicking material. It also introduces significant complexity and cost to the production of the wick.